The present invention relates to a method of reprocessing spent nuclear fuel.
A reprocessing technology practiced at present is the Purex method. The Purex method is that spent nuclear fuel containing uranium and plutonium is dissolved into nitric acid, and the nitric acid solution is in contact with an organic phase which is prepared by diluting an extracting agent of TBP (tributyl phosphate) with dodecane to separate and purify uranium and plutonium utilizing a property of uranium and plutonium moving to the organic phase more easily than most of fission products. The outline of the Purex method is described, for example, in Japanese Patent Application Laid-Open No.9-138297. In the method described in this patent gazette, spent nuclear fuel is dissolved in nitric acid, and then the solution is transferred to a process called as a co-decontamination process, and there, uranium and plutonium are extracted together using the organic solution of diluting TBP with dodecane. The co-extract of organic solution containing uranium and plutonium is transferred to a process called as a distribution process. On the other hand, the fission products are remain in the nitric acid solution which has initially contained the uranium and the plutonium is processed as a high radioactive liquid waste. In the distribution process, the nitric acid solution phase is in contact with the organic solution containing uranium and plutonium, and plutonium is inversely extracted to the nitric acid solution phase by being reduced to trivalent plutonium. Since at that time, uranium remains in the organic phase, uranium and plutonium are separated from each other. The uranium and the plutonium separated from each other in the distribution process are transferred to individual purification processes, and the purified plutonium and the purified uranium obtained through the individual purification processes are to be reused as nuclear fuel.
Japanese Patent Application Laid-Open No.9-138297describes a method in which the Purex reprocessing process is simplified to about 20% of the conventional processes to improve the economic feature, and the liquid waste produced from the processing facility is reduced to a very small amount. This second method derives two inventions from the facts that the reactor core fuel used for a fast breeder reactor requires not so high purity in the uranium and plutonium, and that the blanket fuel used for the fast breeder reactor does not require so high purity in the uranium. The first invention is that the distribution process is eliminated by extracting uranium and plutonium in a ratio suitable for the core fuel of the fast breeder reactor in the inverse extraction process, and that the purification process and the related processes and reagents are eliminated by allowing decreasing of a purity degree index of DF (decontamination factor: a value of a ratio of a radioactivity concentration after processing to a radioactivity concentration before processing). This invention is called as a single cycle method because the reprocessing extraction process comprises only the co-decontamination process. The second invention is that a process called as a crystallization process is provided before the co-decontamination process. In the co-decontamination process, the nitric acid solution having dissolved spent nuclear fuel is cooled to separate part of the amount of uranium through re-crystallization. The extraction process and the inverse extraction process can be made compact because an amount of nuclear fuel material to be processed in the co-decontamination process and in the inverse extraction process can be reduced by removing the excessive uranium by the separation, and because the plutonium and the uranium in the mixed ratio for the core fuel can be easily inverse-extracted in the inverse extraction process. In the crystallization process, the excessive uranium is separated as uranium nitrate. The uranium nitrate can be denitrated and roasting-reduced without being purified to be used as the blanket nuclear fuel for the fast breeder reactor.
The two inventions disclosed in Japanese Patent Application Laid-Open No.9-138297 are effective in the case where the recovered uranium and plutonium is used for the fast breeder reactor fuel. However, the uranium and plutonium obtained through reprocessing is also used in light water reactors in addition to the fast breeder reactors. In order to use the recovered plutonium in an exiting light water reactor, the radioactivity of the fuel must be sufficiently low. This problem can be relatively easily solved by restoring the eliminated purification process in the first invention disclosed in Japanese Patent Application Laid-Open No.9-138297. If the extraction process and the inverse extraction process are made compact by the second invention of Japanese Patent Application Laid-Open No.9-138297, the purification process can be made smaller than that in the conventional Purex method. Therefore, plutonium having a DF nearly equal to a DF in the conventional method can be recovered using a very small reprocessing facility, even restoring the purification process, compared to the conventional method.
Another problem relates to use of recovered uranium in the light water reactor. The recovered uranium contains about 1% of U-235, and the U-235concentration must be enriched up to about 4% in order to reuse the recovered uranium in the light water reactor. The uranium oxide obtained from the reprocessing facility has been converted to a chemical form of volatile UF6, and then the concentration of U-235is increased (enrichment of U-235is performed) through a centrifugal separation method or the like. In order to perform these processes, about 10millions of DF is required. In order to purify the uranium separated in the crystallization process, an equipment having nearly the same size of the equipment for the co-decontamination and purification processes in the original Purex method. Accordingly, there is a problem in that the effect of downsizing the reprocessing facility by separation in the crystallization process is canceled.